U.S. patent number 5,390,360 [Application Number 07/977,361] was granted by the patent office on 1995-02-14 for r.f. communication system interrogation apparatus and method.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Shmuel Katar, Shlomo Scop.
United States Patent |
5,390,360 |
Scop , et al. |
February 14, 1995 |
R.F. communication system interrogation apparatus and method
Abstract
In a radio frequency communication system, a primary unit
interrogates a plurality of secondary units over a radio
communication channel. The primary unit receives a command to
initiate an interrogation cycle over the communication channel. The
primary unit then selects a time delay from a set of time delays,
and transmits an interrogation request to each secondary unit of at
least a subset of the plurality of secondary units in series after
the selected time delay if the channel is free at the end of the
selected time delay. The primary unit then selects the longest time
delay for the next interrogation cycle if a channel grant command
is received, and selects a shorter time delay for the next
interrogation cycle if no channel grant command is received.
Inventors: |
Scop; Shlomo (Netanya,
IL), Katar; Shmuel (Tnuvot, IL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
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Family
ID: |
10706749 |
Appl.
No.: |
07/977,361 |
Filed: |
November 17, 1992 |
Foreign Application Priority Data
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Dec 23, 1991 [GB] |
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9127291 |
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Current U.S.
Class: |
455/516;
340/10.2; 455/507 |
Current CPC
Class: |
H04W
28/18 (20130101); H04W 72/02 (20130101); H04W
74/08 (20130101) |
Current International
Class: |
H04L
12/403 (20060101); H01Q 009/00 () |
Field of
Search: |
;455/34.2,34.1,49.1,51.1,51.2,53.1,54.1,54.2,56.1,38.2
;370/85.2,85.6 ;340/825.54 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0064818 |
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Nov 1982 |
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EP |
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0076880 |
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Apr 1983 |
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EP |
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0094180 |
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Nov 1983 |
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EP |
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2050763 |
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Jan 1981 |
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GB |
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2063011 |
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May 1981 |
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GB |
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Primary Examiner: Urban; Edward F.
Assistant Examiner: Sobutka; Philip J.
Attorney, Agent or Firm: Egan; Wayne J.
Claims
We claim:
1. A communication system comprising a primary unit, first and
second secondary units for communicating with the primary unit over
a radio channel and tertiary units for communicating with the
secondary units over the channel, wherein
the primary unit comprises
means for transmitting a command to initiate contention and
interrogation,
each secondary unit comprises
means for receiving said command to initiate contention and
interrogation,
delay means responsive to reception of said command for delaying
for a period of time selected from a set of time delays,
monitoring means for determining whether the channel is free,
transmitting means for executing an interrogation cycle by
transmitting to at least one of the tertiary units if the channel
is still free at the end of the selected delay period,
delay increase means for increasing the delay for the next
transmission when an interrogation cycle is executed, and
delay decrease means for decreasing the delay for the next
transmission when an interrogation cycle is not executed,
and wherein the time delays of the set of time delays of the first
and second secondary units are different.
2. A system according to claim 1 wherein the time delays of the set
of time delays of the first secondary unit are staggered with
respect to the time delays of the set of time delays of the second
secondary unit.
3. A method of performing interrogation of a plurality of secondary
units by a primary unit over a radio communication channel,
comprising the steps of:
receiving at the primary unit, over the communication channel, a
command to initiate an interrogation cycle,
selecting a time delay from a set of time delays,
transmitting an interrogation request to each secondary unit of at
least a subset of the plurality of secondary units in series after
the selected time delay if the channel is free at the end of the
selected time delay,
selecting the longest time delay for the next interrogation cycle
if a channel grant command is received and
selecting a shorter time delay for the next interrogation cycle if
no channel grant command is received.
4. A method according to claim 3, comprising the step of, at the
primary unit, monitoring the channel, initiating a timer on
detection of free channel and transmitting the interrogation
request after the timer has counted through the time delay.
5. A communication unit for communicating with a plurality of other
units over a radio channel, comprising
means for contending for access to the channel,
means for receiving a command to initiate an interrogation cycle
and
means, responsive to said means for contending for access, for
transmitting to each of the other units in sequence following a
successful contention in response to receipt of the command,
wherein the means for contending comprises:
delay means for delaying initiation of said interrogation cycle for
a period of time selected from a set of time delays,
means for monitoring the channel at the end of said period of time
and
means for determining that contention is successful if the channel
is free at the end of said period of time, and wherein, immediately
following a successful contention, the time delay selected for the
next contention is the longest of the set of time delays, whereas
following an unsuccessful contention, a shorter time delay is
selected for the next contention.
6. A communication system comprising a primary unit, first and
second secondary units for communicating with the primary unit over
a radio channel, tertiary units for communicating with the
secondary units over the channel and at least one repeater,
wherein
the primary unit comprises
means for transmitting a command to initiate contention and
interrogation,
the repeater is arranged to receive said command and retransmit it
to the tertiary units,
each tertiary unit is arranged to respond to said command by
initiating an action,
each secondary unit comprises
means for receiving said command to initiate contention and
interrogation,
delay means responsive to reception of said command for delaying
for a period of time selected from a set of time delays,
monitoring means for determining whether the channel is free,
transmitting means for executing an interrogation cycle by
transmitting to at least one of the tertiary units if the channel
is still free at the end of the selected delay period, and
the tertiary units are arranged to respond to transmissions from
the secondary units by confirming that the action has been
initiated.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method of contending for access to a
communication channel and communications apparatus for
communication on the channel such as supervisory control and data
acquisition (SCADA) apparatus.
SUMMARY OF THE PRIOR ART
In recent years, the radio channels have become very crowded for
voice communication as well as for data communication.
In a communications system such as SCADA system, there may be a
number of master control units located in different places, all
trying to interrogate corresponding remote terminal units (RTUs) on
the same radio frequency, frequently even using the same radio
repeater. A typical application in which this situation is common
is a siren control system. Such a system is illustrated in FIG. 1.
In this figure, there is one main controller 11 that sends a siren
activation control signal to a number of RTUs 16--each having a
siren 17 associated with it--in a large area via a common repeater
12, and also to regional centers 13, 14 and 15. The regional
centers have responsibility for gathering data from a group of
sirens after control. Each regional center has to interrogate its
sirens after activation, to find out if there are any problems with
the activation, and they all have to do this via the same common
repeater 12. Usually this situation has the potential of creating
chaos on the radio channel, since all regional centers will start
the interrogation at the same time, with the result that none of
the regional centers is able to receive answers from its sirens in
a reasonable time.
It would be possible to provide a fixed queue such that one of the
regional centers is always the first to interrogate its RTUs,
another regional center is always the second etc., but this
possible solution has the disadvantage that the last regional
center's operator always has to wait a long time between the siren
activation control signal and the appearance of the results on a
monitor screen. This delay is not acceptable. The absence of
physical lines between the regional centers means that there is no
simple way of synchronizing the centers to allow all operators of
the regional centers to be up-dated in equal time.
There is a need for a communication system which allows an orderly
contention for access to the channel, without introducing undue
delay for response from any particular unit.
SUMMARY OF THE INVENTION
According to the invention, a method of contending for access to a
communication channel is provided comprising the steps of receiving
a command to initiate contention, selecting a time delay from a set
of time delays, transmitting a channel access request after the
selected time delay, selecting the longest time delay for the next
contention if a channel grant command is received and selecting a
shorter time delay for the next contention if no channel grant
command is received.
In this way, a revolving time delay is provided, in which a unit
that is successful in contention goes to the bottom of the queue by
virtue of introducing the longest time delay before it is able to
contend again and a unit that is unsuccessful in contention raises
its place in the queue by virtue of having a shorter time delay
upon the next contention.
The length of delay is related to the position in the queue because
other units with shorter delays have the opportunity to request
access to the channel during the delay period.
In a further aspect of the invention, a communication unit is
provided for communicating with another unit over a channel
comprising means for contending for access to the channel, means
for receiving a channel grant command and means for transmitting to
the other unit on receipt of the command, wherein the means for
contending comprise delay means for delaying contention for a
period of time selected from a set time delays and wherein,
immediately following a successful contention, the time delay
selected for the next contention is the longest of the set of time
delays and wherein, following an unsuccessful contention, a shorter
time delay is selected for the next contention.
Two or more such units (regional centers) may be provided in which
the time delays of the set of time delays of one unit are offset
from, and interspersed between, the time delays of the set(s) of
time delays of the second (and further) unit(s).
The invention also provides a communication system as defined in
the claims.
A preferred embodiment of the invention will now be described by
way of example only with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general SCADA communication system in accordance
with the prior art and suitable for modification in accordance with
the invention.
FIG. 2 shows a central unit ("regional center") 13 of FIG. 1,
incorporating improvements in accordance with the invention.
FIG. 3A and FIG. 3B show an algorithm for implementation in the
central unit of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The operation of the system of FIG. 1 is as follows. The main
central unit 11 issues a command to all the RTUs 16 causing them to
activate their sirens. This command is communicated via the
repeater 12 and directly from the repeater to all the RTUs 16. On
receipt of the command, the sirens 17 start up. At the same time,
the regional centers 13, 14 and 15 receive the command via the
repeater 12 and an interrogation cycle is initiated, in which the
regional centrals 13 interrogate all the RTUs in their respective
regions and take appropriate steps if there are sirens that have
failed to start up. An appropriate step could, for example, be
displaying at a monitor at the regional center 13 the fact that a
siren has failed, prompting the operator to dispatch a team to the
location of that siren to manually alert people in that region of
the warning in question. The regional centers 13 can report back to
the main central unit 11 the results of their interrogation
routines. The interrogation routine is described in greater detail
below with reference to FIG. 3.
Referring to FIG. 2, there is shown a regional center 13 comprising
a transmitter part 20, a receiver part 21, a microprocessor 22, RAM
memory 23 and ROM memory 24. Data is transmitted from the
microprocessor 22 through the transmitter part 20 to an antenna 25,
is received through the antenna and the receiver part 21 to the
remote processor 22. The receiver part 21 has a channel monitor
output 30, which passes to the microprocessor 22. The
microprocessor 22 operates under control of a program stored in ROM
memory 24 and reads data from and writes data to the RAM memory
23.
The RAM memory contains a number of registers, lists etc. Two of
these lists are shown in the figure as RTU list 27 and delay
selector 28. These are described in greater detail below with
reference to FIG. 3.
RAM memory 23 also contains the following set up parameters, that
are set at each individual regional center:
Interrogations window length--WWWW (for example 10 seconds)
Number of regional centers in the system:--CCCC (for example
3).
The algorithm illustrated in FIG. 3A and FIG. 3B is initiated at
step 201 by the siren activation command received from the main
central unit 11 via the repeater 12. An algorithm identical to that
of FIG. 3A and FIG. 3B is initiated in all the regional centers 13,
14 and 15 simultaneously. Before describing the algorithm in
detail, its overall operation is that each regional center has to
wait a different time delay of free RF channel (no transmission on
the radio channel) before it can start its interrogation routine.
When the interrogation routine starts, each regional center
interrogates only as many RTUs as it is able to interrogate in a
predetermined time (referred to as an interrogation "window"--a
cycle of interrogation of a few RTUs). While in the interrogation
window period, each regional center contends for free channel
before transmission by checking the channel monitor 30. When the
channel is free, a regional center interrogates its RTUs one after
the other without any delay. After the interrogation "window" time,
a regional center will have to wait again for a minimum delay time
of free RF channel before it can continue with another
interrogation window. The total time to interrogate all the RTUs
may take a few of these windows.
When a number of regional centers are operating on the same radio
channel, the delay time of each given regional center changes
dynamically after each interrogation window, so each unit's
interrogation window alters.
As an example, a system will be described having three regional
centers and a window time of 10 seconds (sufficient time to
interrogate between 4 to 5 RTUs).
In order to eliminate as far as possible the likelihood of
collisions between regional centers that might start with the same
delay, the delay time before commencement of an interrogation cycle
is, for the first regional center 13, selected from the set: 1.0,
2.0 and 3.0 seconds; for the second regional center the set 1.1,
2.1 and 3.1 seconds; and for the third regional center the set 1.2,
2.2 and 3.2 seconds. Fourth, fifth regional centers can have
further sets of possible time delays following the same pattern.
These exact timing delays are given by way of example only. It
will, of course be understood by one skilled in the art that timing
delays do not have to be "selected" from a table, but can be
implemented by increasing and decreasing a value in appropriate
steps. The principle of the preferred algorithm requires that there
are at least "n" periods of the delay to each regional center, the
number "n" equating to the number of regional centers on the same
frequency. The corresponding delay steps in each regional center
should be staggered vis-a-vis those in other regional centers.
These delays are illustrated in Table 1 as follows:
__________________________________________________________________________
Regional Regional Regional center 13 center 14 center 15 FUNCTION
(1st CENTRAL) (2nd CENTRAL) (3rd CENTRAL) STEP
__________________________________________________________________________
Delay before 1.0 1.1 1.2 101 1st cycle Operation INTERROGATION
waiting waiting 102 Delay before 3.0 1.1 1.2 103 2nd cycle
Operation waiting INTERROGATION waiting 104 Delay before 2.0 3.1
1.2 105 3rd cycle Operation waiting waiting INTERROGATION 106 Delay
before 1.0 2.1 3.2 107 4th cycle Operation INTERROGATION waiting
waiting 108 Delay before 3.0 1.1 2.2 109 5th cycle Operation
waiting INTERROGATION waiting 110 Delay before 2.0 3.1 1.2 111 6th
cycle Operation waiting waiting INTERROGATION 112 Delay before 1.0
2.1 3.2 113 7th cycle Operation INTERROGATION waiting waiting 114
Delay before 3.0 1.1 2.2 115 9th cycle
__________________________________________________________________________
When the regional centers 13, 14 and 15 receive an interrogation
start control command from the main central unit 11, all of them
start their delay timer (step 101 in Table 1). The first regional
center 13 having the one-second delay will be the first one to
start the interrogation routine of its RTUs (step 102 and see below
with reference to FIG. 3). The second and third regional centers 14
and 15, detecting the activity on the radio channel will wait and
will not start their interrogation routines. After 10 seconds (the
time of the first regional center interrogation "window"), the
first regional center 13 finishes its interrogation and changes its
delay time (Table 28 in FIG. 2) to 3 seconds (step 103). The second
delay period starts and after 1.1 seconds, the second regional
center 14 will start its interrogation window (step 104). When this
is finished, the second regional center 14 changes its own delay to
the maximum--3.1 seconds (step 105). Meanwhile, the delay of the
first regional center 13 has changed to two seconds because of the
fact that more than 0.7 of the window time has passed and the first
regional center 13 has detected activity of other central units on
the air, which prevented it from starting another interrogation
window. After the second regional center 14 stops its
interrogation, the delay of the third regional center 15 will be
the shortest--1.2 seconds (step 105), so that it will start its
window of interrogation (step 106). After the third regional center
15 has finished its interrogation window, it changes its own delay
to 3.2 seconds (step 107) and in the meantime, the first regional
center 13 has changed its delay to one second and the second
regional center 14 has changed its delay to 2.1 seconds. Thus, the
first regional center 13 will be the next to start another
interrogation routine (108) and so on.
If only one regional center out of the three wishes to start an
interrogation routine, it will wait three seconds (i.e. the highest
delay time) between every interrogation window and will continue to
interrogate all its RTUs in consecutive windows.
Referring now to FIG. 3, the interrogation routine carried out in
each of the regional centers 13, 14 and 15 is illustrated. The
routine is initiated by reception at the regional center in
question of the initiation command from the main central unit 11
(which is preferably the same command as causes the sirens to start
up). In step 202, the RTU interrogation list 27 (FIG. 2) is reset
and in step 203, the first delay is commenced, according to step
101 of Table 1. During this delay, the regional center's receiver
21 monitors the radio frequency for activity on the channel (204).
If the channel is free, and the first delay is finished (205), the
regional center starts its window time WWWW (206). It checks again
for free channel monitor (step 207) and if the channel is free, the
regional center 13 sends an interrogation request to the first RTU
(step 208). The regional center waits for an answer (step 209) and
after the reception of the answer from the first RTU (210) the
regional center checks whether it was the last RTU on the list
(step 212). If it does not receive an answer (step 210) the
regional center goes through a RTU fail mechanism in steps 224,
223, 211 and 225. If an answer has been received at 210, and it is
not the last RTU on the list (212), the regional center advances
the pointer of the RTUs list 27 to the next RTU that has to be
interrogated (step 213). The regional center checks if the "window"
timer WWWW is finished (step 214) and if it is not, its starts to
interrogate the next RTU on the list from step 207 without delay.
After interrogating a number of RTUs, the "window" timer will have
finished and the delay time of the regional center changes to its
maximum (step 215). This is also shown in Table 1 line 103 for the
first regional center 13. The regional center starts the window
timer (216) and the new delay timer (217). The regional center
checks if it receives anything on the channel during the new delay
time and if it does receive activity, it checks the length of the
activity. If this is more than 0.7 of the "window" time 218, it
means that another regional center is performing its interrogation
cycle, so the regional center 13 in question waits for the other
regional center (e.g. 14) to finish a "window". Thereupon, the
channel becomes free (221) and the regional center changes its own
delay timer to one step shorter (222) as shown in Table 1 line
105). The regional center 13 starts another decision cycle from
step 216, but this time its delay timer is shorter. If at step 217
with the new shorter delay, the regional center does not receive
any other unit, it continues through steps 218 and 219 to 220 and
starts a new interrogation cycle of its own.
Thus an algorithm has been described, implemented in software
routines in each regional center based on a revolving time delay in
which the regional centers synchronize by interleaving the
interrogation requests equally between them, so that of the central
units are able to up-date their databases in equal time.
Moreover, a regional center can enter into contention at any time
and "synchronize" to the other units on the channel and avoid
collisions by virtue of commencing its delay from the time the
channel becomes free and initiating a delay that is at no time the
same as the delay for any other unit. All units commence their
delays at the same time and each unit has a delay that is variable
but always unique.
* * * * *